The present invention relates to a stabilized optical sighting system for mounting on a carrier vehicle which, as it moves, is subjected to disturbing roll, pitch, and yaw movements. A particularly important application is on board ships where such disturbances are practically continuous and can be of large amplitude. Nevertheless, the invention is applicable to any medium subjected to such movements, in particular for making sighting systems that enable panoramic surveillance to be performed, i.e. the line of sight should be maintained at an angle of elevation that is constant relative to the horizon while simultaneously causing the line of sight to rotate about an axis that is vertical. The term "optical" should be understood broadly as covering both infrared sighting and sighting in the visible range.
Stabilized optical sighting systems are known that comprise a sensor mounted on a platform that is kept fixed relative to a geographical frame of reference by mounting the system on gimbals provided with motors controlled on the basis of information provided by a navigation control center. The sensor and the platform supporting it together have a large amount of inertia; that degrades the dynamic behavior of the system and requires motors of considerable power.
Consequently, the invention relates to a stabilized sighting system of a type that reduces the inertia of the moving parts comprising an aiming mirror whose orientation about an elevation axis and about a "circular" axis that is fixed relative to a platform secured to the carrier, is controlled to bring the direction of light received along a sighting line that is determined in a geographical frame of reference to a direction that is constant relative to the support, said constant direction being that of the circular axis in the frequent case of a panoramic system.
Often the aiming or sighting mirror is also steerable about a "lateral" axis which is parallel to the circular axis at zero elevation. This structure makes it possible, in particular, to provide a panoramic sight whose mirror is driven at constant speed about the circular axis, with any disturbing movements that tend to alter the angle of elevation being compensated by controlling both a motor for rotating the mirror about the elevation axis and a motor for rotating it about the lateral axis.
FIG. 1 is a diagram showing the theoretical structure of such a system having a lateral axis, together with the parameters involved in controlling it, and the notation that is used below, this figure not being to scale.
The system comprises a sight proper placed at the top of a mast 10 fixed to the deck of the ship 12. It has a head 14 which is steerable by a motor 15 about a circular axis z.sub.1 perpendicular to the deck and relative to a platform 32 which is fixed to the mast. In FIG. 1, x.sub.1 designates the longitudinal axis of the ship (lubber's line), and y.sub.1 the axis lying in the plane of the deck and extending orthogonally to x.sub.1 and z.sub.1. In the head 14, an aiming member constituted by an aiming mirror 16 is steerable both about an elevation axis 18 perpendicular to the circular axis, and about a lateral axis 20 perpendicular to the elevation axis 18. Rotation about the elevation axis 18 is controlled by an elevation motor 22. The output shaft of this motor carries a support 24 both for the lateral axis 20 and for the lateral motor 26 which controls rotation of the aiming mirror about the lateral axis. A deflecting optical assembly 28 deflects the light path so that light leaves the sight substantially along the circular axis. The deflection is generally through 90.degree.. The beam of light coming from the direction of the sighting line 30 is thus successively reflected by the aiming mirror 16 and deflected by the assembly 28.
The sighting line 30 can be defined by a true! elevation angle Si and by an azimuth angle Az in a geographical frame of reference xyz.
The laws for controlling rotation about the circular, elevation, and lateral axes are much more complex than the laws for controlling a platform to keep it fixed relative to a geographical frame of reference, in particular because the lateral axis is moved by the elevation axis. The rate of rotation to be imparted about the circular axis z.sub.1 relative to the ship differs from the azimuth rate to be imparted relative to the terrestrial frame of reference xyz because of the roll, pitching, and yaw movements of the ship. In order to perform panoramic scanning at constant elevation and at substantially constant azimuth rate, the steering angles about the circular, elevation, and lateral axes must all vary continuously. Computing them in real time by successive approximation methods requires very high computation power, given the inevitable defects of the system.